CRITICAL STRESS CORROSION CRACKING POTENTIALS OF STAINLESS STEELS IN DILUTE CHLORIDE SOLUTIONS

摘要

Stainless steels are susceptible to stress corrosion cracking (SCC) in wide range of environments. SCC occurs predominantly when stainless steels are exposed to chloride containing environments. The concentration of these chloride ions vary with different industrial applications from 10ppm in cooling water to 22,000ppm (seawater). This study was focused on low chloride concentrations and is relevant to steam turbines and cooling water systems.The effect of applied potential (E_(app)) on (SCC) behaviour of stainless steels (12%Cr -25%Cr, PREN 11 - 43) was investigated using the slow strain rate technique (SSRT) in an autoclave with controlled potentials, using a three electrode cell and potentiostat. The tests were conducted at 130°C in a solution containing 15-30ppm chloride and 8ppm dissolved oxygen. The results showed that the reduction in ultimate tensile strength (UTS) and the reduction of area (ROA) varied with the applied potential. A critical SCC potential (E_(scc)) was found to exist in a narrow potential range for each of the materials testedFractographic observations on fractured specimens in the scanning electron microscope (SEM) showed that the loss of ductility, indicated by the decrease in ROA was linked to the initiation, coalescence and propagation of cracks for all materials. Depending on themicrostructure the forms and causes of SCC in all these materials was different. Safe operating limits in dilute chloride conditions were fully determined for all grades investigated in this study.The Escc value for given steel was related to the pitting resistance equivalent number (PREN) and varied by more than 0.80V across the range of steels studied. Increasing Cr, Mo, N content increases SCC resistance, but all steels crack above E_(scc). Most SCC problems for higher PREN steels at low chloride levels can be controlled by ensuring Eservice < E_P (Pitting potential). The use of high PREN steels could eliminate chloride SCC. This paper also emphasis the observed corrosion behavior by evaluating local crystallographic information, and plastic strain determined using Electron Back Scattered Diffraction (EBSD) across range of materials studied.